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The second method is the empirical orthogonal function-based index. This index was first proposed by Kao & Yu (2009), who classified patterns with SST empirical orthogonal functions higher than 1σ and durations of 3 months as a CP El Niño. The El Niño Modoki index (EMI) (Ashok et al. 2007), as applied by Yoon et al. (2013), is the third method. In this method, the CP El Niño is defined as having a 3-month moving average EMI higher than 0.66 and duration of 6 months from September to February of the following year. Finally, we used the mean CP index (Kao & Yu 2009) to classify CP El Niño as years when the CP index was higher than 1σ. Table 1 provides more detailed information on the CP El Niño events selected in this study. CP El Niño events can be classified into three different groups (prolonged-decaying pattern, abrupt-decaying pattern, and symmetric-decaying pattern) according to the evolution patterns of the SSTs in the central region of the Pacific Ocean (Yu & Kim 2010).

Table 1

El Niño classification identified by different definitions

YearType of El NiñoClassification methods
1977/1978 CP A, B, C, D 
1987/1988 CP A, B, D 
1990/1991 CP A, C, D 
1991/1992 MIX*(CP/EP) or CP A*, B, C, D 
1994/1995 CP A, B, C, D 
2002/2003 MIX*(CP/EP) or CP A*, B, C, D 
2004/2005 CP A, B, C, D 
2009/2010 CP A, B, D 
YearType of El NiñoClassification methods
1977/1978 CP A, B, C, D 
1987/1988 CP A, B, D 
1990/1991 CP A, C, D 
1991/1992 MIX*(CP/EP) or CP A*, B, C, D 
1994/1995 CP A, B, C, D 
2002/2003 MIX*(CP/EP) or CP A*, B, C, D 
2004/2005 CP A, B, C, D 
2009/2010 CP A, B, D 

CP, Central Pacific El Niño; EP, Eastern Pacific El Niño.

El Niño-type classification: A: NINO method (Yeh et al. 2009: Nino 4 > Nino 3 (DJF); Kim et al. 2009: Nino 3 < 1σ, Nino 4 > 1σ (ASO)). B: empirical orthogonal function (EOF)-based index (Kao & Yu 2009: SST EOF > 1σ for more than 3 consecutive months). C: EMI method (Yoon et al. 2013: EMI (3-month moving average) >0.66 and for more than 6 consecutive months (SONDJF)). D: NWP index (Ren & Jin 2011: Nwp > 1σ and for more than 3 consecutive months (DJF)).

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